Crystalline polyesters have been recently noticed as a resin binder suitable for improving a low-temperature fusing property of toners because they have a high compatibility with non-crystalline polyesters and are readily dispersed therein unlike other crystalline resins such as polyethylene, and exhibit a clear melting point owing to their crystalline moieties.
JP-A-2004-191927 (“JP-A” means unexamined published Japanese patent publication) discloses a toner for developing an electrostatic image which contains at least a crystalline polyester and an amorphous polymer as a resin binder and whose surface is coated with a surface layer containing the amorphous polymer as a main component, in which a content of the crystalline polyester is in the range of from 30 to 80% by weight, a proportion of the crystalline polyester contained in an outermost surface of the toner for developing an electrostatic image is 15 atomic % or less, and an average thickness of the surface layer is not less than 0.01 μm and not more than 0.5 μm, for the purpose of providing a toner for developing an electrostatic image which can be fused at a low temperature and has a sufficient charging property.
JP-A-2008-158197 discloses a process for producing a toner for developing an electrostatic image which includes at least an aggregating step of mixing a dispersion of non-crystalline polyester resin particles, a dispersion of crystalline polyester resin particles, a dispersion of colorant particles and a dispersion of releasing agent particles to form aggregated particles containing the non-crystalline polyester resin particles, the crystalline polyester resin particles, the colorant particles and the releasing agent particles; and a fusing and coalescing step of heating the aggregated particles to a temperature not lower than a glass transition temperature of the non-crystalline polyester resin to fuse and coalesce the aggregated particles, in which a mixture of the non-crystalline polyester resin and the crystalline polyester resin satisfies a specific molecular weight, for the purpose of providing a toner for developing an electrostatic image which is capable of forming a stable image for a long period of time.
The resin binder for toners which contains a crystalline polyester obtained using a carboxylic acid component containing an aliphatic dicarboxylic acid compound having 8 to 12 carbon atoms as a main component is excellent in low-temperature fusing property, but tends to be deteriorated in pressure storage stability and environmental stability.
JP-A-2004-191927 and JP-A-2008-158197 both disclose a so-called chemical toner or method using an aqueous dispersion of a crystalline polyester, but fail to specify the above problems and provide a means for solving the problems.